$\cos\frac{\pi }{5}+\cos\frac{2\pi }{5}+\cos\frac{3\pi }{5}+\cos\frac{4\pi }{5}-\cos\frac{5\pi }{5}-\cos\frac{6\pi }{5}-\cos\frac{7\pi }{5}-\cos\frac{8\pi }{5}-\cos\frac{9\pi }{5}$ bằng?
4
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